This invention relates to an apparatus for processing mail articles such as postcards and letters, and more particularly to a stamp detector used in a mail processing apparatus such as a mail cancelling and/or facing apparatus.
Luminescent stamps that emit fluorescence or phosphorescence have been widely used as postage stamps in, for example, Europe and U.S.A. In mail cancelling and/or facing apparatus, therefore, the fluorescence or phosphorescence emitted from the postage stamps is utilized to detect the postage stamps. In the conventional mail cancelling and facing apparatus, as disclosed in the U.S. Pat. No. 3,938,435 entitled AUTOMATIC MAIL PROCESSING APPARATUS issued to Suda et al., the mail articles are taken one by one from a mail feeding portion and are transferred through a transport path. The stamp on the mail article is then detected by stamp detectors disposed along, and on both sides of, the transport path. Then, the stamp is cancelled by cancellers which are disposed on both sides of the transport path and are selectively actuated on the basis of the results of the stamp detection. The mail articles are positioned to face with the stamps in the same relative position, in response to the positions of the detected postage stamps.
In order to improve the mail processing efficiency in the mail cancelling and facing apparatuses of this kind, the sensitivity of the stamp detector has been increased to detect the stamps which emit a weak luminescence. However, when the stamp having large intensity of emitting fluorescence or phosphorescence is put on a thin mail article, such as a thin postcard, the emitted luminescence would penetrate to the reverse side of that article. Hence, both of the pair of stamp detectors disposed on opposite sides of the transport path would produce stamp detection outputs. In this case, the mail article is not processed and is rejected as an abnormal mail article. For this reason, the processing efficiency, or detection ratio, of the conventional mail cancelling and facing apparatus would drop if a large number of thin mail articles are contained in a batch of mail articles. If the sensitivity of the stamp detector is reduced to prevent such an erroneous operation, the stamp having a low luminescent intensity can not be detected and consequently, the subsequent cancelling and/or facing operation can not be effected. This also results in the deterioration of the processing efficiency of the apparatus.